The present invention is directed to a silicon controlled rectifier (SCR) nickel metal hydride (NiMh) battery charging circuit. More particularly, the present invention relates to a SCR battery charging circuit particularly adapted to charge the kind of battery, such as the NiMh type battery, which has a critical temperature limit and a need for a certain amount of self-generated heat to reach a full capacity of recharge. The invention is further directed to a battery charging circuit which generates for a given range of ambient temperature a profile of SCR set voltage, i.e. the voltage at which the SCR becomes non-conducting, dependent on both cell temperature, under the influence of the prevailing ambient temperature, and cell voltage in a manner designed to accommodate to the aforesaid limitations.
SCR-type charging circuits have been in manufacture for years. In a typical circuit the voltage potential at the SCR""s cathode must be less than the voltage potential at the SCR""s gate for the SCR to conduct, or turn to the xe2x80x9cONxe2x80x9d state. The gate potential can be set at a desired level for a specific type of battery and the charger will return energy to the battery until that set potential is reached, at which point the SCR will automatically turn xe2x80x9cOFFxe2x80x9d. However, in the typical SCR type charging circuit the set potential is not made to vary as in the present invention dependent on changes in both the cell temperature and cell voltage.
Here it should be noted that batteries of the kind being referred to are made up of one or more xe2x80x9ccellsxe2x80x9d which are spoken of as comprising a xe2x80x9cbattery packxe2x80x9d. Therefore, the terms xe2x80x9ccellxe2x80x9d, xe2x80x9ccellsxe2x80x9d, xe2x80x9cbatteryxe2x80x9d and xe2x80x9cbattery packxe2x80x9d are sometimes used interchangeably.
The SCR circuit has worked well for charging SLA (sealed lead acid) and similar types of battery products which do not require critical cell temperature control under charge. More recently, new battery systems and in particular, nickel metal hydride (NiMh) batteries have become popular since NiMh batteries have an advantage in that they do not contain cadmium, as do nickel cadmium (NiCad) batteries, cadmium being a non-desired pollutant. Furthermore, the NiMh cells which make up a NiMh battery have an energy density about 80% greater than that of the cells which make up a NiCad battery thus allowing for longer equipment run time, or, smaller and lighter equipment with the same run time.
There are special concerns for NiMh batteries under charge. As previously mentioned, NiMh batteries have a critical upper temperature limit. In addition, they require a certain amount of self-generated heat to reach a full capacity of recharge. These two temperature conditions are relatively close together, requiring a charging system, which can allow one to be met, and, prevent the other from being reached. Attempts have been made to meet the NiMh battery charging control requirements. However, such attempts have led to very costly control means. The high cost of the control means has limited the market for both NiMh batteries and NiMh battery rechargers. Uncontrolled and relatively inexpensive charging circuits, like those presently available for recharging NiCad type batteries, can be used for recharging NiMh batteries, but such NiCad chargers if used for recharging a NiMh battery risk shortening the life of the NiMh battery because of excessive heat.
As background for later description reference is next made to FIG. 1A which is a schematic representation of a known SCR-type battery charger. The voltage potential at the cathode xe2x80x9cCxe2x80x9d of the SCR shown in FIG. 1A must be less than the voltage potential at the gate xe2x80x9cGxe2x80x9d for the SCR to conduct, or turn to the xe2x80x9cONxe2x80x9d state. The gate potential can be set at a desired level for a specific type of battery and the charger will return energy to the battery until that set potential is reached, at which point the SCR will automatically turn xe2x80x9cOFF.xe2x80x9d In the circuit of FIG. 1A, Zener Diode (ZD), resistors R1, R2, and potentiometer P are used to provide a controlled and consistent voltage to the gate G of the SCR over a wide range of AC input voltage to the transformer T. However, the FIG. 1A circuit lacks means for cell temperature control.
Two types of rectification are shown in schematic FIGS. 1A and 1B. FIG. 1A shows a Full Wave (FW) center tap rectifier, and FIG. 1B illustrates an alternative FW bridge rectifier either of which can be used without concern to the rest of the circuit. While the portion of the circuit connected to the rectifiers is not shown in FIG. 1B, it should be understood that such portion is similar to that of FIG. 1A. As previously stated, either type of rectification can be used with either of the circuits of the invention, but for simplification, only the FW center tap rectifier will be discussed by way of example throughout the rest of the description. The boxes BX, with an xe2x80x9cXxe2x80x9d therein on either side of the SCR as shown in FIG. 1A, schematically represent various accessory items that can be used in the circuit, but do not affect its operation. The accessories may include, by way of example, accessories such as a state of charge display, overload protection circuits, and impedance control.
Here it should be recognized that circuits of the kind illustrated in FIGS. 1A and 1B will not charge NiMh batteries with proper results. Use of such circuits for recharging NiMh batteries will cause the cells to overheat and experience a reduced cell life. On the other hand, NiCad batteries are sometimes recharged by use of a FIG. 1A or 1B circuit.
As further background, reference is next made to FIG. 2 which is a schematic diagram of another known type of battery charging circuit that helps to overcome the previously mentioned excessive heating problem particularly when charging NiMh batteries. The circuit illustrated in FIG. 2 is similar to that of FIG. 1A, but includes the addition of resistor R3 and negative temperature coefficient (NTC) thermistor which operates both to control the end of voltage, and to provide a means by which to utilize cell temperature as a means for terminating charge. Thus, the circuit of FIG. 2, unlike the circuit of FIG. 1A, does provide a form of cell temperature control under charge. Those skilled in the art will understand that when making reference to a circuit such as shown in FIG. 2, the phrase xe2x80x9cend of voltagexe2x80x9d is conventionally used interchangeably with the term xe2x80x9cset voltagexe2x80x9d and that the xe2x80x9cgatexe2x80x9d voltage is not exactly the true xe2x80x9csetxe2x80x9d voltage but for practical purposes is treated as being equal to the true set voltage. The NTC thermistor has a variable resistance that changes in value to the inverse of the temperature being sensed. In other words, its resistance decreases with increasing temperature and visa versa. In order to function properly in a battery charging circuit, the NTC device is bonded to a cell in the battery pack to accurately sense battery temperature.
With a proper set of charge circuit components, it is recognized that the battery pack shown in FIG. 2 could achieve a desired rise in temperature to attain an approximate but less than full charge particularly of a NiMh battery, and the gate voltage potential could be set such that the charging circuit will cut off without reaching an unacceptable battery temperature when the battery is operating in a given ambient temperature. Of particular significance however is the fact that the FIG. 2 type circuit only allows an approximate but less than full capacity to be returned to a NiMh battery. That is, the charging circuit necessarily cuts off before the battery is fully charged. Furthermore, the time for recharge, with a circuit of the FIG. 2 type, can vary from about 1.5 hours up to about 12 hours depending on the charge current delivered by the recharger. The remaining small percentage, say for example 5%, of returned capacity is not obtainable in a well defined time period because of the random nature of the charge pulses in the nearly xe2x80x9cOFFxe2x80x9d state. Here it also should be noted that when the charger turns xe2x80x9cOFFxe2x80x9d in the FIG. 2 type circuit, it will pop xe2x80x9cONxe2x80x9d and xe2x80x9cOFFxe2x80x9d at a random rate while attempting to return full capacity to the battery. In the FIG. 2 circuit, the SCR is hard OFF at the switch over point. Under this condition, the battery back emf will drift down until the set temperature/voltage point is reached which will xe2x80x9cfirexe2x80x9d the SCR or turn it to the ON state. In a short time, the battery is brought back to the cut off point and the SCR again turns OFF which causes the xe2x80x9cpoppingxe2x80x9d effect. Thus, for the reasons stated, full capacity is not truly obtainable with a circuit of the FIG. 2 type.
Users of battery charging circuits and particularly for those used for charging NiMh batteries oftentimes desire some form of charge indicator and mention has been made above of use of a state of charge display in circuits such as illustrated as BX in FIG. 1A. However, the above-mentioned xe2x80x9cpoppingxe2x80x9d ON and OFF is truly random in the FIG. 2 type circuit and predicting what the end of charge looks like on a display is difficult to obtain. Some charging circuits of the FIG. 2 type incorporating a charge indicator tend to register an xe2x80x9cOFFxe2x80x9d state most of the time, while other circuits tend to indicate xe2x80x9cONxe2x80x9d or xe2x80x9cOFFxe2x80x9d at a fast cycle. Thus, while use of a NTC thermistor in the type charging circuit shown in FIG. 2 offers advantages accurate indication of the charge state is difficult to obtain with FIG. 2 type charging circuit.
Another approach to recharging batteries is found in U.S. Pat. No. 4,424,476 which describes a circuit in which the end of charging is made to depend upon the cell temperature exceeding the ambient temperature by a predetermined amount. The ""476 patent is also recognized as illustrating a slow rate charge path established by a resistor in shunt with the SCR and designed to maintain the battery in overcharge above ambient. The circuit of the ""476 patent however has several disadvantages among which is that of not being able to compensate for heat contributed by the charger itself to the ambient temperature. Another disadvantage arises in trying to determine a reasonably accurate ambient temperature since wherever the thermistor T-1 of the ""476 patent is located it will be effected by such factors as direct air from an air conditioning unit, an open window, sunlight through a window and other extraneous ambient heat factors. Similarly the same conditions mentioned for T-1, i.e.: air conditioner open window, etc., will apply to battery thermistor T-2. While other disadvantages of the ""476 patent recharging circuit could be pointed out, it is sufficient to observe that unlike the recharging circuit of the present invention, the ""476 patent recharging circuit is not capable of accurately switching from a relatively fast charge rate to a relatively slow charge rate upon the battery being charged achieving a charge of about 95% of capacity.
When the circuitry of the ""476 patent is compared to the battery charging circuitry of the present invention, it will be seen that in the circuitry of the invention, unlike in the circuitry of the ""476 patent, a voltage corresponding to the temperature of the battery operating within a given range of ambient temperature is constantly compared with the battery terminal voltage. When this comparison indicates a preset condition, the invention circuitry causes the SCR of the invention circuitry to become non-conducting and charging of the battery to its full capacity to become dependent on a preset trickle current.
Thus, it becomes the primary object of the invention to provide a controlled, relatively fast charger for charging to substantially full capacity the type of battery characterized by having during recharging both a critical temperature limit and a need for a certain amount of self-generated heat in order to be able to reach a substantially full capacity of recharge.
While the primary object is that stated above as applied to any kind of battery of the described character, a particular object of the invention is that of providing an improved battery recharging circuit for a NiMh type battery.
While further recognizing the primary object as being that stated above, another object is to provide a battery recharging circuit which facilitates display of a substantially full charge.
Other objects will become apparent from the following detailed description and through practice of the invention.
The invention resides in the discovery with respect to recharging a battery of the type characterized by having during recharging both a critical temperature limit and a need for a certain amount of self-generated heat that both requirements can be met by incorporating in a recharging circuit of the FIG. 2 type means for permitting throughout a relatively wide range of ambient temperature a trickle current to flow through a shunt around the SCR after the SCR is turned OFF and at a level which causes the SCR to remain OFF while the battery being charged is permitted to reach a substantially full charge without exceeding the battery""s critical temperature limit and simultaneously to self-generate such amount of heat as is required for the battery being charged to reach a substantially full charge.
The invention further resides in the discovery that the trickle current as recited above can be established in such a way as to eliminate intermittent turning ON and OFF or so-called xe2x80x9cpoppingxe2x80x9d of the SCR and permit use of a charge indicator operative to indicate when the battery being charged has reached a substantially full charge.
The invention charger circuitry enables the gate voltage to be set at a desired level for a specific type of battery operating at a particular battery temperature and the charger to return energy to the battery until that set voltage is reached across the battery terminals as indicated by the battery back emf (electromotive force) reaching a value across the battery terminals at the particular battery temperature equal to the set voltage.
The ambient operating temperature limit of the invention circuitry shown in FIGS. 3, 4, and 10 has been assumed for practical reasons to limit use of the battery charging circuitry of the invention to general office or indoor use, not intended for outdoor applications. The assumed range of ambient temperature under these conditions is thus assumed to vary from about 10xc2x0 C. to about 40xc2x0 C. By limiting the intended use in this way, the battery charging control circuitry of the invention can be made at reasonable cost and the invention circuitry can become of greater economical value to the marketplace.